(CNN) -- By the time Clarence Jones reached him, the Rev. Martin Luther King Jr. was in bad shape.

He was unshaven, dirty and dejected. King had spent several days alone in solitary confinement with no mattress in a filthy dark jail cell in Birmingham, Alabama.

"Take this out of here," King whispered as he grabbed Jones' belt and stuffed bawled-up newspapers and toilet tissue down his pants.

Jones, King's lawyer, wondered if King was starting to lose it. He didn't pay attention to what King had given him -- it was just a mish-mash of words and arrows scribbled on bits of paper.

"Not until five days later did I actually read a mimeographed copy of the letter," says Jones. "To be honest with you, I was more worried about bail money, not what he had written."

Millions of people have since read what Jones first ignored. As the nation commemorates the 50th anniversary of King's "Letter from Birmingham Jail" on Tuesday, the document has become an American epistle. It's considered a classic defense of civil disobedience.

But those who see King's letter as just a tract on nonviolent resistance make the same mistake King's lawyer made: They miss what's special about something that's right in front of their eyes, some King scholars say.

The letter is one of the most intimate snapshots of a King most people don't know: An angry black man who once hated white people and, according to one scholar, was more dangerous than Malcolm X, a man King admired.

"Before everything else, (the letter) is a black man's cry of pain, anger and defiance," says Jonathan Rieder, author of the just-released "Gospel of Freedom," which looks at the "furious truth teller" revealed in King's classic letter.

King's blackness -- his fierce racial pride, his distinctively black Christian faith and his belief that most whites were "unconscious racists" -- is on full display in his letter, scholars say. The anger that drove King's letter would become more prominent in the speeches King gave until, literally, his last hours, Rieder says.

"If there was a YouTube in 1968 and some of King's sermons would have been captured, he would have been seen as a Jeremiah Wright," says Rieder, invoking the name of President Obama's fiery former pastor.

Rieder says King's dual nature -- the "crossover King" with the refined, professorial manner, and the private, even sarcastic King -- take turns on center stage in the letter.

Around trusted friends, King was a man who smoked cigarettes, cracked people up with his impersonations of pompous black preachers, and once abruptly ended a meeting by telling his staff as he headed to a concert: "I'm sorry, y'all, James Brown is on. I'm gone."

"King was a bad-ass," says Rieder, a sociology professor at Barnard College in New York who spent a decade listening to King's private and public recordings. Rieder also wrote, "The Word of the Lord is Upon Me," a book that revealed King's backstage discussions with friends and colleagues.

Yet the "crossover King" -- the man who never embraced black separatism or abandoned nonviolence -- is just as authentic, Rieder says, and his moral passion has inspired nonviolence activism around the globe.

"King's words -- the bristling at those who tell the oppressed to 'wait for a more convenient season' ... has resonated among freedom fighters long after the 'Letter' was written," Rieder writes in "Gospel of Freedom."

Jones, the lawyer who smuggled the letter for King, stopped wondering about King's emotional stability after he sat down to read a typed-up version of King's notes.

Jones had given King's notes to a 17-year-old secretary at the Southern Christian Leadership Conference, the organization King led. The secretary deciphered King's scrawl and produced the written version. (She complained that King could preach, but his handwritten notes sure were hard to follow.)

"When I read it, I said, 'Oh my God, it's a masterpiece," says Jones, author of the 2011 book, "Behind the Dream: The Making of the Speech that Transformed America."

"He didn't have a book around him. He did this all out of his head. With all due respect to many things he's written, including the 'I Have a Dream' speech, the letter is one of the most profound (examples of) literature created in the 20th century," says Jones, who helped write King's "Dream" speech. Today Jones is a visiting professor at Stanford University's Martin Luther King Jr. Research and Education Institute in California.

King's letter, though, didn't receive much attention when it was published by the Quakers as a pamphlet in May. It did not produce a breakthrough moment in King's campaign against segregation in Birmingham -- that moment came when the Rev. James Bevel, one of King's lieutenants, suggested using children as demonstrators -- and was initially ignored by the local and national press.

But in summer 1963, the letter started to gain traction, appearing in the Christian Century, Atlantic Monthly and Saturday Evening Post magazines. Rieder says it gained its widest audience as a chapter in King's popular 1964 book, "Why We Can't Wait."

One minister was so inspired by reading King's letter in the Christian Century that he wrote, "If the canon of the Holy Scripture were not closed, I would nominate it ... as an addition to the Epistles in the best tradition of the Pauline prison letters."

The clergymen King addressed in his letter had a more mixed reaction, Rieder wrote in his book. Some thought it worsened racial tensions, while another called it a "vicious document" and complained that he received letters throughout his life asking if he was still a bigot.

King on edge

The letter was written at a moment when King thought he had failed.

He and others had organized a massive protest in one of the most violently segregationist cities in the South. The local Ku Klux Klan was one of the most violent in the nation and had infiltrated the police department. The KKK had been suspected in so many bombings of Birmingham's black community that the city had been dubbed "Bombingham." (Four black girls attending a black church in Birmingham would be killed by a bomb five months after King's letter.)

But the protests faltered because activists couldn't summon enough participants and were running out of bail money for those who had been arrested. King decided he needed to do something dramatic. He provoked his arrest by leading a demonstration on April 12, 1963, Good Friday.

King's boldness may have been forced by events outside Birmingham. He had become a national figure eight years earlier after leading the successful bus boycott in Montgomery, Alabama, that brought Rosa Parks to fame. Yet he'd had no major victories since then, and a younger generation of activists -- sit-in protesters and Freedom Riders -- were grabbing headlines and questioning King's toughness.

"They had taken away the initiative from King," says Clayborne Carson, editor of the multi-volume "The Papers of Martin Luther King, Jr.," one of the most comprehensive collections of King's speeches and writings. King's late widow, Coretta Scott King, selected Carson to edit King's papers.

"Nearly everything that happens, nothing of it has anything to do with King," says Carson, director of the King institute at Stanford. "King needed a victory desperately."

King's desperation deepened after he was placed in solitary confinement in the Birmingham jail. He hated being alone. He depended on the company of people for emotional support after his many arrests. He also had been scarred by an earlier experience when he was driven to an isolated jail in rural Georgia where he thought he was going to be killed.

King also had been virtually isolated by his own community. Only about five black churches in Birmingham allowed King to use their churches for mass meetings. The rest wanted nothing to do with him, says Rieder.

"A lot of them were being cautious about politics," he says. "They were part of the professional classes and didn't want to rock the boat. And some of them didn't like the idea of the big man coming in to tell them what to do."

Depressed and angry and alone in jail, King read an ad that had been placed in a Birmingham newspaper by eight moderate white clergymen. The newspaper had been smuggled to King while he was in jail. In the ad, the clergymen called King an outside agitator and lawbreaker and counseled him to wait.

King's struggle with anger

King didn't take their advice. Scribbling in the margins of the newspaper or on whatever paper he could find, he became the angry prophet. He unloaded on the clergymen.

Writing only from memory, he deftly cited Socrates, St. Augustine, the Jewish philosopher Martin Buber and the theologian Paul Tillich. He was schooling the clergymen on their own faith.

At one point in the letter, he responded to criticism that he was advocating breaking the segregationist laws in Birmingham. How could a minister tell people to break the law?

King said there was a difference between just and unjust laws. An unjust law is one that a dominant group is not willing to follow itself.

"We should never forget that everything Adolf Hitler did in Germany was 'legal,'" King wrote. "It was 'illegal' to aid and comfort a Jew in Hitler's Germany."

The emotional fire in the letter comes when King speaks as a black man, some scholars say.

King once detested white people because of his experiences growing up in the segregated South, Rieder says. Though King grew up in the black middle class of Atlanta, he experienced all sorts of racial humiliation and saw blacks treated with viciousness.

"He went through a period of hatred toward whites, and it took him some time to get over it," Rieder says. "He would say that when he saw Malcom X on television, there would be times when he would feel that old bitterness rising."

King wrote about that hatred as a graduate student at Crozer Theological Seminary, Rieder says. In a paper entitled, "Autobiography of Religious Development," King wrote about how shocked he was to hear how his parents had been insulted by whites. King was once ordered to give up his bus seat for a white person and cursed as a "nigger."

"As my parents discussed some of the tragedies ... I was determined to hate every white person," King wrote. "As I grew older and older this feeling continued to grow. ... I did not conquer this anti-white feeling until I entered college."

That bitterness pervades King's letter.

King wrote that it was easy for white people to tell blacks that they were moving too fast. But when you "have seen vicious mobs lynch your mothers and fathers at will ... when your first name becomes 'nigger' and your middle name becomes 'boy' ... you will understand why we find it difficult to wait."

King declared in the letter that the "great stumbling block" toward black freedom wasn't the white racist "but the white moderate."

Rieder says King identified with black anger so much that when race riots spread across America in the mid-1960s, he refused to demonize black rioters. King once said that a riot "is the language of the unheard."

"It was his Christianity that wouldn't allow him to hate, but he wasn't alien to those feelings," Rieder says. "He wasn't above it. He never looked down on it."

King was particularly angry at the white church. He expected support from Southern white churches, but said in the letter that most "have remained silent behind the anesthetizing security of stained glass windows."

He even wonders in his letter, as he reminisces about passing beautiful white Southern churches during his travels, if they share the same faith:

"What kind of people worship here?" he asks. "Who is their God?"

James Cone, the father of black liberation theology, says King was so angry because he loved his people.

"Love is passion," says Cone, a professor at Union Theological Seminary in New York. "If you love somebody and they are hurt, you are hurt, too. King identified with black people who had experienced injustice and humiliation. You can't look at that and not get mad. King's anger was expressed precisely in the letter."

Speaking the language of his people

King spoke the language of white theologians in the letter, but it showed he was bilingual -- that he was also fluent in the language of the black church, some scholars say.

In his book, "Martin & Malcolm & America: A Dream or a Nightmare," Cone talks about the fierce pride King had in his black heritage.

King talked about taking pride in his kinky hair, once told a group of ministers that Jesus "was not a white man" (he said Jesus' color was irrelevant), and in his famous letter and elsewhere King linked the civil rights movement with the black liberation movements in Africa, Cone says. (King attended Ghana's independence celebration in 1957 and Nigeria's in 1960.)

In the letter, King boasts about being the "son, grandson, and the great-grandson" of black preachers. Years before afros, dashikis and black power, King was invoking black pride.

"Before the pilgrims landed at Plymouth, we were here," King wrote in the Birmingham letter. "Before the pen of Jefferson etched the majestic words of the Declaration of Independence ... we were here. For more than two centuries our forebears labored in this country without wages; they made cotton king; they built the homes of their masters while suffering gross injustice ... and yet out of a bottomless vitality they continued to thrive."

King's affirmation of the black church was a pattern. He often cited songs and phrases that were distinctive to the black church. He closed his "I Have a Dream" speech with the words from a Negro spiritual. The last sermon he delivered the night before his assassination -- his "I've been to the mountaintop" speech -- was built on riffs off the Exodus story, a biblical narrative favored by slaves.

"When your back is against the wall, you can't have a borrowed faith," Cone says. "It's not your intellect that gives you courage. What gives you courage is that which has come out of your own history, that which has brought you to the place where you are."

Cone says King's anger was more threatening to America than Malcolm X's.

"White people didn't like Malcolm, but they could tolerate him because Malcolm wasn't organizing black people in a way that would challenge the function of the government," Cone says. "King did."

Though King's letter made little difference in the Birmingham campaign, it did set up one of his greatest public moments.

"If the Birmingham campaign would have failed, there would not have been an "I Have a Dream" speech because he would not have been invited to give the "I Have a Dream" speech," says Carson, editor of the King papers.

King was given the coveted closing spot at the 1963 March on Washington, where he was free to improvise the "I Have a Dream" speech because he had no time restrictions, Carson says.

That speech is still more celebrated than King's Birmingham letter.

Rieder, the "Gospel of Freedom" author, says he knows why:

"It's King as a dreamer with black and white children holding hands," Rieder says. "It becomes part of the myth of a post-racial society: 'Aren't we fine people?'"

The letter, he says, undermines America's investment in its feel-good celebration of King. Many Americans aren't ready to meet the King revealed in his epic letter.

That King isn't a dreamer, Rieder says, but someone else: The angry black man who wonders what kind of God some white people follow.